Proposal: Improved Type Inference for Generic Instance Creation (original) (raw)

Joseph D. Darcy Joe.Darcy at Sun.COM
Sat Feb 28 00:02:29 PST 2009

Jeremy Manson wrote:

Hey Neal,

I think you might have missed where I asked for an example where G1 and G2 might have different numbers of type parameters or they might have corresponding parameters in a different order. I'm probably missing a very obvious example, and I'm sure you have an example in mind. On Fri, Feb 27, 2009 at 11:33 PM, Neal Gafter <neal at gafter.com> wrote:

Adding conversions doesn't make type inference work. Each conversion that you want to affect type inference has to be accommodated specifically in the type inference algorithm.

I wasn't suggesting that adding conversions would make type inference work. I was suggesting that the conversion could be done in place of the type inference. I disagree with your suggestion that this should fail: List<?> list = new ArrayList<>(); // should fail The equivalent code using static factories works just fine. It isn't clear why this feature should behave differently, or what part of your specification makes it fail. Basically, because I specifically disallowed wildcards under the sample spec. See the second bullet point under "Semantics and Compilation". However, I wasn't convinced that that was a good idea either, which is why I put it under Additional Features (at the end). The described technique under that header would make this example work (probably -- I'd have to double-check the JLS). I don't think supporting casts adds value to the feature. You can already assign, for example, from ArrayList to List without a cast. That was where Josh was leaning when I asked him about it, too. The reason I thought it made sense was just for consistency's sake; basically, it is the only one of the five conversion contexts where this technique might work, but it doesn't. For assignments and method parameters, it will work, and for String and numeric conversions, it doesn't make any sense. Casting was the only one left. If you want to hear about the approach that Joe is working on, I'll be happy to describe it in person at your convenience (with Joe, if he wants).

I would definitely like to hear about it! How close is Joe to finishing a draft?

Joe hasn't started a draft!

Neal and I discussed some implementation strategies that could also guide a specification. I'll send out more in the next few days.

Good night,

-Joe

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